(250b) Elastohydrodynamic Lubrication of Robotic and Human Fingertips on Soft Micropatterned Substrates

With the recent increase in online remote interactions, the concept of tuning haptic (touch) perception using soft materials is becoming important in many areas of engineering. In many of these scenarios involving haptic perception, a thin interfacial liquid film is present between two deformable surfaces, leading to elastohydrodynamic lubrication (EHL). We report systematic studies of EHL friction on micropatterned soft substrates under three different sliding conditions with a stress-controlled tribometer, robotic fingers, and human fingers. Mixtures of water and glycerol are used to generate fully flooded EHL conditions. We characterized more than 50 patterned soft tribopairs comprising elastomers, thermosets, hydrogel on our tribometer and discovered that micropatterns induces a critical transition on the macroscopic friction coefficient. To capture the experimental critical friction coefficient, we develop a theory that combines surface geometry, Reynolds’ equation, and elastic deformation to provide physical insights behind the multiphysics phenomenon. Our theory not only applies to well-controlled experimental setup but also suits less controllable situations with robotic fingers and human fingers. The parameter-free agreement of fingertip data with our theory demonstrates that the EHL friction of soft, textured substrates are tuned by varying pattern geometry, elasticity, and thin fluid film properties. This framework provides new tools that may be applied in product design and manufacturing, and knowledge that may inform the engineering of robot hands, epidermal electronics, and haptic technologies.